The invention relates to systems and methods for correcting bone deformities using external fixators, and in particular to using systems to plan and optimize bone deformity correction treatment with external fixators.
Patients with bone deformities suffer from a reduced quality of life: they may suffer from difficulties in standing, walking, or using limbs. Bone deformities can be congenital, or the result of a fracture that did not heal properly. These deformities can include axial, sagittal, or coronal plane deformities, translational or rotational deformities and mal-union or non-union deformities, or, in complex cases, more than one type of deformity.
The bone deformities are often treated with surgery. For example, surgeons may use metal implants to improve the geometry of a deformed bone; however, inert metal implants are not as flexible in their ability to reform natural bone in something close to normal anatomical geometry. As an alternative, surgeons may perform an osteotomy—a cut in the bone—and then attach an external fixator to support the growing bone while the bone deformity is corrected. The Taylor Spatial Frame (TSF) is a commonly-used external fixator comprising rings interconnected by struts. After the osteotomy, the surgeon may insert pins through the superior and inferior sections of the bone. These pins are attached to external rings so that one ring is roughly perpendicular to the superior section of the bone, and the other ring is roughly perpendicular to the inferior section of the bone. The surgeon may attach adjustable struts to these rings so that the rings are held together by the struts. Each strut has a predetermined attachment point to each ring. Because the rings are each fixed to a section of bone, and because the rings are now joined by flexible struts, the bones can be moved with six degrees of freedom relative to each other.
After the surgery to attach these rings and struts, a surgeon may take orthogonal x-rays of the apparatus on the patient's leg. The surgeon may make a number of measurements from the x-ray images, including distances and angles of both the tibia and the rings and struts. The surgeon may then use the numerical measurements to calculate the bone correction needed and prescribe for the patient the length of each strut to be adjusted each day. Typically, daily adjustments will be made, realigning the sections of the bone at a rate that allows new bone to form ultimately yielding natural bone in a geometry that comes close to normal anatomy and function.
Such calculations are usually time-consuming and usually rely on the assumptions that each ring is perfectly perpendicular to the bone segment to which it is attached. This may require a surgeon to spend extra time in the operating room to assure that each ring is perpendicular to each corresponding bone segment. If a ring is not perpendicular to its corresponding bone segment, error will be entered and the resulting prescription for strut adjustments will not be accurate.
This system of two rings and six struts may be chosen for several reasons. First, the system allows a surgeon to move the two bone segments with six degrees of freedom relative to each other, thereby giving the surgeon the freedom to treat many types of deformities. The system may be strong enough to support body weight so that a patient can be ambulatory while healing occurs.
The shortcomings of the procedure include the difficulty and lack of accuracy in using a ruler and protractor on an x-ray print-out, or digital system not related to the prescription calculation program, to measure distances and angles, the amount of time involved in performing all the calculations needed to generate the patient prescription, and the surgical difficulty in positioning the external fixator exactly with respect to the patient's bone.
It would thus be desirable to develop a system that would allow easy and accurate measurements of bones and bone deformities, and easily generate accurate prescriptions for strut lengths for the bone correction treatment.